In electronic communications between a transmitter and a receiver, LFPSs are sometimes used to initiate transactions. For example, in a Universal Serial Bus (USB) link, an LFPS may be a handshake signal between the two ends of the link before initiating high-frequency data transmission. An LFPS can be used to bring back a far-end device from a power-down state. LFPSs may be sent by a high-speed transmitter, e.g., by sending many identical digital bits, although other methods can be used.
Although most of the time the LFPS is sent continuously until detection is achieved, some information may also be exchanged using intermittent LFPSs. For example, LFPS “ping” signals are short LFPS sequences that are used to change test patterns sent by the far-end device in certain testing modes. The low-frequency nature of the LFPS is meant to make it easy to detect, without requiring sophisticated bit recovery. Intersymbol interference (ISI) is minimized by the low frequency of the LFPS.
Conventional LFPS detection circuits, which employ an analog filter, are highly susceptible to detection errors. For example, an analog low-pass filter may include a resistor (R) and a capacitor (C), both of which are subject to manufacturing or temperature induced variations that change the corner frequency (commonly defined as the frequency where the received signal is attenuated by 3 decibels or dB). Increasing the corner frequency leads to an amplitude drop at the filter output, while decreasing the corner frequency increases the amplitude. Thus, the filter may erroneously block LFPSs or erroneously pass high-frequency signals.
Another limitation of analog filters is that they may contribute to false detection of LFPSs by passing intermediate and low-frequency components of a high-frequency signal that varies in instantaneous frequency. Thus, a detector circuit operating on the output of an analog filter might register the envelope of a high-frequency signal as being an LFPS. To prevent such false detection, the R or C components may have to be increased so that the corner frequency is correspondingly decreased. A USB specification or other communication protocol may require a minimum frequency to be detected (e.g., a frequency associated with short ping signals). Therefore, the lowering of the corner frequency is limited by the need to comply with the minimum frequency.
Yet another limitation of the analog approach is that analog filters need to balance detection time and noise tolerance. While waiting for a steady state response, detection errors may occur during a transient phase. For example, a short LFPS pulse may not reach an expected amplitude, so that setting of an appropriate detection threshold may be difficult.